The top layer of human skin, the stratum corneum (SC) consists of protein enriched corneocytes embedded in a lipid matrix. The major function of the structure, as the body's protective barrier to the environment that prevents the loss of water and nutrients from within, is predominantly determined by the levels, composition and structure of the SC lipids. The SC lipids also influence the mechanical and desquamatory (skin cell shedding) activities of the SC. Abnormalities in the SC lipids can occur in connection with various conditions such as aged or photo damaged skin, or in connection with xerosis (a condition of abnormal dryness) such as during winter months.
Replenishing SC lipids by topical application of hydrophobic compounds is an approach that has been used with limited success. A longer lasting approach is to exploit the robust epidermal lipid biosynthetic pathways of the viable epidermis using natural extracts or compounds that upregulate the body's natural production of these lipids, and particularly a class of critically important SC lipids known as ceramides. Nicotinamide, for example, has been reported to increase the synthesis of ceramides, major constituents of lipids present as lamellar sheets in intercellular spaces of the SC. Br. J. Dermatol. (2000 September) 143(3):524-31. In addition, certain isomers of lactic acid are also reported to stimulate ceramide biosynthesis. Arch. Dermatol. Res. (1996) 288: 383-390.
It is well recognized that ceramides are functionally and structurally distinct from lipids that are present in deeper layers (i.e., the hypodermis) of the skin. For example, while ceramides are a class of polar lipids that play a role in cell membrane structures to enhance skin barrier function, the deeper hypodermal lipids are non-polar lipids whose function relates to energy storage, thermal insulation, and protection of internal organs from mechanical injury.
It has been reported that Southernwood extract is suitable for “fat restructuring” and stimulating adipogenesis, presumably by affecting the non-polar lipids of hypodermis. See US 2009/0285868 A1. Surprisingly, the inventors have now found that Southernwood extract, acting on an entirely different biological pathway, is suitable to enhance the biosynthesis of ceramides. Accordingly, the inventors have found that Southernwood extracts are remarkably suitable for treating completely different need states, and completely different skin, than what is known in the art. Specifically, the inventors have found that Southernwood extract is surprisingly suitable for the topical treatment of visibly dry, e.g., xerotic skin.
Furthermore, the inventors have now found that Southernwood extracts, when combined with particular amine compounds, provide surprisingly improved ability to enhance ceramide synthesis as well as maintain hydration (water levels) in the upper layers of the stratum corneum.